The proposed experiments focus on interactions between maternal and zygotic components during cellularization and gastrulation in Drosophila development. We will study two genes [folded gastrulation (=fog) and concertina (=cta)] which are required for normal formation of the posterior midgut, characterizing the morphological phenotypes of both loci in terms of cell shape and morphogenetic movements. By injecting wild type cytoplasm into eggs from cta femase, we will test whether cta= activity is localized to the same regions of the embryo known to required fog expression. Both cat and fog will be cloned. In situ hybridization and antibodies to their respective protein products will be used to determine whether the gene products are localized only to the posterior regions of the embryo affected by each mutant. The probes will also be used to define in molecular terms the epistatic relationships between the two loci. We will also study two zygotically active loci (nullo 6F1-2 and halo, 22A) which cause abnormalities in actin and lipid distribution early during cellularization. The phenotype of mutants and wild type embryos processes will be characterized morphologically and genetic mosaics will be used to determine the autonomy of the phenotypes and the specificity of the gene products for embryogenesis. Interactions between both loci and maternal effect mutations shown to affect cellularization will be analyzed. The nullo locus will be cloned and the distribution of its protein product during the establishment of the hexagonal actin arrays will be described.